Method and apparatus for producing inorganic fullerene-like nanoparticles

ABSTRACT

A process and apparatus are presented for obtaining inorganic fullerene-like nanostructures. A metal oxide is evaporated at predetermined temperature conditions, and is swept towards a reacting zone, to which first and second gas phase reacting agents are concurrently swept. The evaporated metal oxide thus interacts with the first reacting agent and is converted into metal suboxide nanoparticles in the gas phase. The condensing metal suboxide nanoparticles interact with the second reacting agent in the gas phase resulting in substantially pure phase of the inorganic fullerene-like nanoparticles.

FIELD OF THE INVENTION

This invention relates to a chemical method and apparatus for producing inorganic fullerene-like nanoparticles.

BACKGROUND OF THE INVENTION

Inorganic fullerene-like nanoparticles and nanotubes are attractive due to their unique crystallographic morphology and their interesting physical properties. Molybdenum disulfide belongs to a class of solid lubricants useful in vacuum, space and other applications where liquids are impractical to use.

It is known to obtain carbon fullerenes and nanotubes by a synthetic process consisting of condensation of molecular clusters from the vapor phase. This is disclosed for example in the following publication:

-   -   (1) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science         of Fullerenes and Carbon Nanotubes; Academic Press, INC, 1996;         pp 1–6, 110–116;     -   (2) Flagen, R. C.; Lunden, M. M. Materials Sc. Eng. A 1995, 204,         113;     -   (3) Deppert, K.; Nielsch, K.; Magnusson, M. H.; Kruis, F. E.;         Fissan, H. Nanostructured Materials 1998, 10, 565; and     -   (4) Kruis, F. E.; Fissan, H.; Peled, A. J. Aerosol Sci. 1998,         29, 511.

According to this technique, a hot vapor is quenched and entrained by a flowing inert gas. Nanoclusters are obtained by an adiabatic expansion of the vapor leading to a cooling down of the clusters-inert gas vapor and its condensation. Fullerene-related nanoparticles are derived from materials with a layered structure.

There are three main types of fullerene-related particles: fullerenes (C₆₀,C₇₀,etc.); nested-fullerene nanoparticles (onions), and nanotubes. Analogous fullerene-like nanoparticles can be obtained from a number of inorganic materials with layered structure, which are known as inorganic fullerene-like materials-IF.

Fullerenes can be produced-from carbon-rich vapor or plasma, which can be obtained using laser ablation, arc discharge or resistive heating of graphite targets, as disclosed in the above publication (4), as well as in the following publications:

-   -   (5) Kroto, H. W.; Heath, J. R.; O'Brien, S. C.; Curl, R. F.;         Smally, R. E. Nature 1985, 318, 162;     -   (6) Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, R.         Nature 1990, 347, 354.

Techniques for obtaining fullerene-like nanoparticles of MoS₂ from MoS₂ powder subjecting the powder to e-beam irradiation and laser ablation are disclosed in the following publications:

-   -   (7) Jose-Yacaman, M.; Lorez, H.; Santiago, P.; Galvan, D. H.;         Garzon, I. L.; Reyes, A. Appl. Phys. Lett. 1996, 69, 8, 1065;     -   (8) Parilla, P. A.; Dillon, A. C.; Jones, K. M.; Rider, G.;         Schulz, D. L.; Ginley, D. S.; Heben, M. J. Nanure 1999, 397,         114.

According to another known techniques, short electrical pulses from the tip of a scanning tunneling microscope over a film consisting of amorphous MoS₃ nanoparticles, can be used to cause the formation of nanoparticles with a closed MoS₂ shell (IF), a few molecular layers thick, and amorphous MoS₃ core. This is disclosed for example in the following publication:

-   -   (9) Homyonfer, M.; Mastai, Y; Hershfinkel, M.; Volterra, V;         Hutchison, J. L.; Tenne, R. J. Am. Chem. Soc. 1996, 118, 33,         7804.

A technique of obtaining closed cages and nanotubes of NiCl₂ has been recently developed and disclosed in the publication:

-   -   (10) Rosenfeld-Hacohen, Y, Grinbaum, E., Sloan, J.,         Hutchison, J. L. & Tenne, R., Nature 1998, 395, 336.

According to the above technique, heating of NiCI₂ to 960° C. in a reducing atmosphere is utilized.

As recently reported by Chhowalla and Amaratunga in Nature 407, 164 (2000), clusters of hollow fullerene-like MoS₂ “onions” can be produced in the form of thin films.

IF nanoparticles, including nanotubes, may also be obtained by chemical methods. The first synthesis of IF—MS₂(M=Mo,W) is based on the sulfidization of the respective amorphous MO₃ thin films in a reducing atmosphere at elevated temperatures (˜850° C.). This technique has been developed by the inventor of the present application, and are disclosed in the following publications:

-   -   (11) Tenne, R; Margulis, L.; Genut, M.; Hodes, G. Nature 1992,         360, 444;     -   (12) Margulis, L.; Salitra, G.; Tenne, R.; Talianker, M. Nature         1993, 365, 113;     -   (13) Hershfinkel, M.; Gheber, L. A.; Volterra, V; Hutchison, J.         L.; Margulis, L.; Tenne, R. J. Am. Chem. Soc. 1994, 116, 1914.

As further found by the inventors of the present application, the synthesis of IF-MoS₂ including MoS₂ nanotubes can be carried out by using molybdenum oxide powder instead of a thin film precursor. This is disclosed in the following publication:

-   -   (14) Feldman, Y.; Wasserman, E.; Srolovitz, D. J.; Tenne, R.         Science 1995, 267, 222.

However, the above synthesis resulted in miniscule amounts of the nanoparticles and a limited size control.

More recently, macroscopic quantities of IF—WS₂ and WS₂ nanotubes were obtained from a powder of tungsten oxide nanoparticles. These techniques are disclosed in the following publications:

-   -   (15) Feldman, Y.; Frey, G. L.; Homyonfer, M.; Lyakhovitskaya,         V.; Margulis, L.; Cohen, H.; Hodes, G.; Hutchison, J. L.;         Tenne, R. J Am. Chem. Soc. 1996, 118, 5362.     -   (16) Rothschild, A.; Frey, G. L.; Homyonfer, M.; Tenne, R.;         Rappaport, M. Mat. Res. Innovat. 1999, 3, 145.

The mechanism for the synthesis of IF-MS₂ (M=Mo, W) from the oxide powder that adequately describes the growth of IF-WS₂ from WO₃ nanoparticles, as found by the inventors of the present application, was more specifically disclosed in the article:

-   -   (17) Feldman, Y.; Lyakhovitskaya, V.; Tenne, R. J. Am. Chem.         Soc. 1998, 120, 4176.

According to this mechanism, within the first few seconds of a chemical reaction, an encapsulate consisting of a skin of monomolecular MS₂ layer or two with a suboxide core, is formed. It was shown that the kinetics of the sulfidization/reduction processes on the surface of the oxide nanoparticles vary strongly with temperature. Only in the temperature range 700–850° C. do the kinetics of the reaction allow sufficiently rapid generation of an absolutely closed spherical sulfide monolayer. This sulfide monolayer averts the fusion of the oxide nanoparticles into micron-size particles and promotes the growth of concentric spherical layers, characterizing fullerene-like structures. Later on, the suboxide core is progressively converted into IF sulfide layers by a slow diffusion-controlled reaction. Consequently, the size and shape of the IF particles are determined by the size and shape of the incipient oxide nanoparticles. It is important to note that the size of the oxide nanoparticles must not exceed 300 nm, otherwise 2H platelets of the respective sulfide are obtained.

MoS₂ and WS₂ nanotubes were also obtained by chemical vapor transport of the MS₂ powder with iodine as a transporting agent, as disclosed in the following articles:

-   -   (18) Remskar. M.; Skraba, Z.; Cleton, F.; Sanjines, R.; Levy, F.         Appl. Phys. Lett. 1996, 69, 351.     -   (19) Remskar, M.; Skraba, Z.; Regula, M.; Ballif, C.; Sanjines,         R.; Levy, F. Adv. Mater. 1998, 10, 246.

In an another approach, aquaeous solution of ammounium thiomolybdate was soaked into a porous alumina template. This is disclosed in the article:

-   -   (20) Zelenski, C. M.; Dorhout, P. K. J: Am. Chem. Soc. 1998,         120, 734.

Subsequent annealing led to the formation of MoS₂ nanotubes, which were isolated by dissolving the alumina matrix with KOH solution.

More recently, sonochemical methods have been used for the synthesis of IF structures from various inorganic compounds, including Tl₂O. This is known from the following publication:

-   -   (21) (a) Mastai, Y; Homyonfer, M.; Gedanken, A.; Hodes, G. Adv.         Mater. 1999, 11, 1010; (b) Avivi, S.; Mastai, Y; Hodes, G.;         Gedanken, A. J. Am. Chem. Soc. 1999, 121, 4196; (c) Avivi, S.,         Mastai, Y, Gedanken, A. J. Am. Chem. Soc., 2000, 122, p.4331.

SUMMARY OF THE INVENTION

There is a need in the art to facilitate the production of inorganic fullerene-like nanostructure by providing a novel chemical method and apparatus therefor.

It was found by the inventors that the previously developed mechanism for the synthesis of IF—WS₂ from the oxide powder, cannot be used as is for obtaining molybdenum fullerene-like nanoparticles.

The main concept of the present invention is directed to providing a process for obtaining pure phase of inorganic fullerene-like nanostructures having well defined size. Relatively narrow size distribution of the resulting inorganic fullerene-like nanostructures can be obtained.

The process utilized in the present invention is a chemical process in the gas phase, and is suitable for mass production of inorganic fullerene-like nanostructures. This process is based on reduction of metal oxide in the gas phase.

There is thus provided according to one aspect of the present invention, a process for obtaining inorganic fullerene-like nanostructures, the method comprising the steps of:

-   -   (a) evaporating a metal oxide at predetermined temperature         conditions;     -   (b) driving a flow of the evaporated metal oxide towards a         reacting zone by means of an inert carrier;     -   (c) supplying a first reacting agent in the gas phase into the         reacting zone, thereby causing an interaction between the         evaporated metal oxide and the first reacting agent to convert         the evaporated metal oxide into metal suboxide nanoparticles in         the gas phase, resulting in condensation of the suboxide         nanoparticles; and     -   (d) supplying at least one second reacting agent in the gas         phase into the reacting zone in the path of the condensing metal         suboxide nanoparticles, thereby causing an interaction between         said at least one second reacting agent and the condensing metal         suboxide nanoparticles, and obtaining substantially pure phase         of the so-produced inorganic fullerene-like nanoparticles.

The first and second reacting agents interact with the evaporated metal oxide at different subsequent sub-zones of the reacting zone with respect to the direction of flow of the evaporated metal oxide. This is due to different diffusion paths of the first and second reacting agents, which are of different molecular weights, and consequently, one diffuses faster than the other.

In one embodiment of the invention, the first and second reacting agents are provided to the reacting zone with the flow direction opposite to the direction of flow of the evaporated metal oxide, where the evaporated metal oxide first interacts with the first reacting agent. This is achieved either by the fact that the propagation of the first reacting agent is to a larger extent than that of the second reacting agent or by constructing different paths for the first and second reacting gases, where the former is directed into the stream of evaporated metal oxide

In step (c), the conversion of the evaporated metal oxide into metal suboxide nanoparticles is done by reduction. Reduction occurs by a chemical reaction of the vaporized metal oxide and hydrogen gas (i.e., the first reacting agent). As a result of the reduction process, the metal suboxide nanoparticles are condensed, since their vapor pressure is lower than that of the metal oxide powder in the vapor phase.

Preferably, the second reacting agent is H₂S, the interaction thereby causing sulfidization of the metal suboxide nanoparticles, resulting in the production of the metal sulfide nanoparticles. It should, however, be noted that, generally H₂Se, or any other volatile organic sulfur or selenium compound, or mixtures thereof may be used as the second reacting agents.

According to a preferred embodiment of the invention, the metal oxide powder is MoO₃. Consequently, reduction results in the formation of MoO_(3−x), clusters. In step (b), the temperature is in the range of from about 710° C. to about 830° C. In step (d), the temperature is about 650–950° C.

The size of the nanoparticles produced by the method of the present invention can generally be in the range of 5–300 nm. It is important to note that the nanoparticles produced with certain selected temperature conditions of the above process are characterized by narrow size distribution, e.g. 5–20 nm.

It should be noted that in order to remove the oxygen from the produced IF nanoparticles precipitating on a ceramic filter, and to introduce sulfur or selenium (or the mixtures thereof, as the case may be), an annealing of the precipitating material is carried out. For this purposes, the precipitating material is further heated to a temperature in the range 850° C.–950° C. for several hours (about 2–4 hours).

According to another aspect of the invention, there is provided an apparatus for obtaining inorganic fullerene-like nanostructures, the apparatus comprising a chemical reactor and temperature control means accommodated outside the chemical reactor and operable to provide predetermined temperature conditions for different zones of the reactor thereinside, wherein the chemical reactor comprises:

-   -   (i) a first flow line for passage of a metal oxide powder         therethrough towards a reacting zone, the temperature control of         the interior of the first flow defining an evaporation zone         within the first flow line for evaporating the metal oxide         powder on its flow towards the reacting zone     -   (ii) a second flow line for passage of first and second         reacting-agents towards the reacting zone, where they interact         with the evaporated metal oxide, the first and second flow lines         being oriented with respect to each other to define         predetermined diffusion paths of the first and second reacting         agents within said zone of interaction with the evaporated metal         oxide to thereby enable obtaining the inorganic fullerene-like         nanostructures; and     -   (iii) a collector part for collecting the inorganic         fullerene-like nanostructures.

The apparatus comprises one or more inlet for flowing the reacting agents through the second flow line. As for the metal oxide powder, it may either be predominantly kept inside the first flow line prior to commencing its evaporation and flow, or may be flown continuously or discretely through an additional inlet means.

The temperature control means includes heating elements appropriately accommodated in the vicinity of the flow line.

According to a preferred embodiment of the invention, the first and second flow lines are implemented as two concentric tubes. These tubes are preferably mounted one inside the other, the interior of the central tube serving as the first flow line, and the interior of the surrounding tube serving as the second flow line. This construction may be installed inside a third tube, which serves for discharging the reaction gases. The central tube (the first flow line) may be shorter than the second surrounding tube, and the resulting product precipitates at the bottom of the second tube (the second flow line).

According to another embodiment, the first and second flow lines while being implemented as concentric tubes may and may not be arranged as one inside the other, but being just aligned in a spaced-apart parallel relationship. In this case, the first and second reacting agents input the second flow line in a direction opposite to the flow direction of the evaporated metal oxide towards the reacting zone.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

FIG. 1A is a schematic illustration of a chemical reactor according to one embodiment of the invention;

FIG. 1B more specifically illustrates the main flow line of the reactor of FIG.

FIG. 2 schematically illustrates the components' distribution within the reacting zone of the flow line of FIG. 1B;

FIG. 3A graphically illustrates the experimental results showing the effects of the flow rate of the first reacting agent on the diffusion length and IF size;

FIG. 3B graphically illustrates the experimental results showing the effects of the flow rates of the first and second reacting agents on the diffusion length and IF size;

FIG. 4 illustrates the experimental results showing the effect of the concentration of the first reacting agent on the diffusion length and IF yield;

FIG. 5 illustrates the experimental results showing the effect of the concentration of the second reacting agent on the diffusion length and IF yield;

FIG. 6 illustrates the experimental results showing the evaporation rate of Molybdenum oxide as a function of the temperature;

FIG. 7A illustrates the experimental results showing the effect of Molybdenum oxide evaporation rate on the entire diffusion length of the first and second reacting agents and on IF yield;

FIG. 7B illustrates the experimental results showing the effect of Molybdenum oxide evaporation rate on the size of the so-obtained IF-MoS₂;

FIG. 7C illustrates the experimental results showing the effect of Molybdenum oxide evaporation rate on the diffusion length of the first reacting agent;

FIG. 8 illustrates images of the IF nanoparticles with two different sizes;

FIG. 9 illustrates a Transmission Electron Microscope (TEM) image of a plurality of IF nanoparticles;

FIG. 10A is a schematic illustration of a chemical reactor according to another embodiment of the invention;

FIG. 10B is a schematic illustration of the gas velocity profile in tube B of the chemical reactor of FIG. 10A; and

FIGS. 11 and 12 are schematic illustrations of two more examples of a chemical reactor according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

More specifically, the present invention is used with Molybdenum Oxide for production of inorganic fullerene-like (IF) MoS₂ nanoparticles, and is therefore described below with respect to this application. Generally, a chemical process of the invention for the synthesis of inorganic fullerene-like MoS₂ is based on the above-indicated mechanism as found by the inventors (with respect to WS₂). This mechanism consists of three main steps: evaporation of a metal oxide powder, condensation of the oxide vapor to obtain the reduced metal suboxide nanoparticles in the gas phase, and sulfidization of the reduced metal suboxide nanoparticles.

Although there are many common features in the synthesis of the IF phase of MOS₂ and WS₂, they differ in some important details. MoO₃ powder evaporates at temperatures above 700° C., while WO₃ does not sublime below 1400° C. Therefore, at the prevailing reaction temperatures (around 850° C.), the sulfidization of MoO₃ and WO₃ powders occur by the gas-phase and solid-gas reactions, respectively.

The above actually means that, in order to obtain pure phase and desired size (preferably narrow size distribution) of the IF nanoparticles of a specific metal, appropriate temperature and other chemical process conditions should be selected to meet the requirements of the specific resulting product production.

For example, with respect to the synthesis of IF-WS₂, prior to this process, the precursor-oxide-nanoparticles of a desirable size and shape must be prepared. In the case of IF-MoS₂, as will be described more specifically further below, partial reduction of the trioxide molecular clusters leads to their condensation to form suboxide nanoparticles, which serve as an intermediate in the IF-MoS₂ synthesis. The process requires a precise control over the reaction products (yield and size) and reproducible synthesis of these nanoparticles. This step is a prerequisite for a scale-up of the IF-MoS₂ phase synthesis.

Referring to FIG. 1A, there is illustrated an apparatus 1 used for production of IF-MoS₂ nanostructure. The apparatus comprises of a chemical reactor 10 that consists of three concentric quartz tubes: an inner (central) tube 10 a (constituting a first flow line), an intermediate tube 10 b (constituting a second flow line) and an external tube 10 c. A reacting zone RZ starts inside the inner tube 10 a at its distal end and extends towards a bottom part of the reactor, where the resulting product is collected (precipitates) on a ceramic filter CF. MoO₃ powder is maintained in the tube 10 a within an evaporation zone EZ above the reacting zone RZ. The reactor 10 is associated with three vessels (not shown) containing, respectively, N₂ (constituting an inert carrier), H₂ (constituting a first reacting agent) and H₂S (constituting a second reacting agent). These materials are sequentially fed to the reactor 10 through flow controllers 20 and 22. The controller 20 controls the flow of N₂ towards the evaporation and reaction zones EZ and RZ through the inner tube 10 a, and the controller 22 controls the flows of N₂, H₂ and H₂S towards the reacting zone RZ through the intermediate tube 10 b. To this end, the reactor 10 is formed with three inlet opening IN₁, IN₂ and IN₃ for supplying the N₂, H₂ and H₂S. It should be noted, although not specifically shown, that an additional inlet may be provided in the reactor 10 for supplying additional amount of MoO₃ powder in a continuous flow or by discrete portions thereof. The connection between the interiors of the central and intermediate tubes 10 a and 10 b is carried out by means of three nozzles, generally at 23. Also formed in the reactor at its upper part is an outlet opening OUT for discharging the reaction gases. The geometry of the flow lines inside the reactor 10 is shown in the figure in a self-explanatory manner.

Further provided in the apparatus 1 are heating elements—upper and lower heating elements 24 and 26 (constituting a temperature control means) accommodated outside the reactor 10. The heating elements are operated to provide required temperature conditions of the entire process. More specifically, the upper element 24 serves for maintaining the temperature conditions required for the evaporation of the metal oxide, while the lower element 26 serves for providing and maintaining the temperature condition required for the chemical reactions. Practically, the three-tube quartz reactor was installed inside a tube like oven (i.e., two heating elements). The temperature profile of the oven was accurately determined with a precision of ±1° C. along the designed quartz reactor. A small bucket 32 is accommodated in the tube 10 a for the MoO₃ powder (in the amount of 1 g in the present example) to be placed therein. The height and size of the bucket 32 were varied until an optimal configuration was obtained.

In the present example of FIG. 1A, the mutual orientation and dimensions of the flow lines 10 a and 10 b enable initial flow of the first and second reacting agents into the flow line 10 b in the same direction as the flow of the metal oxide towards the reaction zone RZ.

Following is the description of an example of using the apparatus 1 (with the flow lines' dimensions as shown in FIG. 1A) to obtain IF nanoparticles.

EXAMPLE 1

In the present example, typically 1 g of MoO₃ powder was placed in the bucket 32 where the amount of the evaporated powder in each experiment is in the range of 100–105 mg. Such a portion does not influence appreciably the level of the oxide in the bucket. Therefore, the rate of the oxide evaporation from the bucket was time-independent. For a long experiment (that lasts more than 4 hours), the temperature T₀ of the bucket 32 has to be increased during the experiment in order to keep the evaporation rate constant.

The flow rate of N₂ (J_(N2)) in the inner tube 10 a (i.e., to drive the evaporated metal oxide towards the reacting zone) was controlled by the flow controller 20 (FIG. 1A). The flow of the other gases (H₂S, forming gas H₂/N₂ and N₂) needed for the reactions to occur in the reacting zone RZ were provided by the flow controllers 22. The forming gas was supplied with different flow rates J_(FG) and volume percents C_(H2) of hydrogen (C_(H2)=1÷8). The H₂S concentration (C_(H2S)) was varied from 1% to 4%. Finer control of the H₂ concentration was achieved by dilution of the forming gas with N₂. Manometers were appropriately installed in the flow lines for determining and controlling the gas pressures. A significant increase in the gas pressure downstream of the flow controller 20 was indicative of the obstruction of nozzles 23 by the reaction product.

The oxide evaporation temperature T₀ was varied from 710° C. to 810° C., keeping the nozzles'temperature T_(n) (where the sulfidization process starts) almost constant (800° C±10° C.). The product was collected on the ceramic filter CF with tortuous paths. Typically, about 150mg of MoO₃ powder was sublimed, from which 50–70mg of IF-MoS₂ were obtained as a single and isolated (pure) phase during 5–6 hours. The rest of the metal material was transformed into various phases, which were deposited on the quartz substrate 30 (not shown) inside the tube 10 a. The quartz substrate 30 was placed inside the tube 10 a so as to extend along the distal end of the tube. Hence, the reaction products, during their flow towards the bottom part of the reactor, adhere to the surface of the quartz substrate and can be analyzed. It should be understood that the quartz substrate 30 serves for the experimental purposes only, and is not a part of the chemical reactor, and the above parameters present one specific example only.

The reaction products were analyzed by X-ray powder diffractometer model Rigaku Rotaflex RU-200B with CuK_(α) anode (1.54178A), transmission electron microscope Philips model CM-120, and scanning electron microscope JEOL 6400, equipped with energy dispersion spectrometer Link model ISIS. Samples for transmission electron microscopy (TEM) analysis were prepared by suspending the powder in ethanol, using ultrasonic bath, and subsequent dripping of the suspension on the grid and drying. The size of the nanoparticles was determined by TEM analysis, based on statistics over a few hundreds nanoparticles for each sample.

The following is the description of a model capable of explaining the growth mechanism for fullerene-like MoS₂ nanoparticles in the gas phase underlying the method of the present invention:

The first step in the synthesis of IF-MoS₂ is the evaporation of MoO₃. The vapor phase of MoO₃ consists predominantly of the molecular clusters: Mo₃O₉, Mo₄O₁₂ and MosO₁₅. The cluster Mo₃O₉, which has a hexagonal ring structure, is the most stable one, and, consequently, it is the most volatile cluster of the three. In the next step, these clusters condense into larger oxide nanoparticles.

According to the prior art IF growth mechanism (developed by the inventors of the present invention), oxide nanoparticles 5–300 nm in size are necessary as a precursor for the synthesis of IF-MoS₂.

In the present process the temperature increases from step to step, having a positive gradient along the molecular clusters flow. Therefore, the classical method for nanoparticles synthesis by cooling and condensation of the vapor phase could not be invoked to explain the current growth mechanism. However, condensation can be provoked also by a chemical reaction, in which case the vapor pressure of the product is lower than that of the precursor in the vapor-phase. This mechanism explains the formation of oxide nanoparticles from the (MoO₃)₃ clusters in the invented method.

A gas phase reaction of diffused hydrogen with Mo₃O₉ molecular clusters leads to their reduction into molybdenum suboxide. Being non-volatile at the present working temperatures, the reduced clusters condense into suboxide nanoparticles, MoO_(3−x). Once the condensation step is completed, i.e., nanoparticles of 5–300 nm in size have been obtained; they are allowed to encounter H₂S and the conversion into fullerene-like MoS₂ nanoparticles commences.

Reference is now made to FIG. 1B which more specifically illustrates the reactions that take place in the reactor 10, with reference thus also being made to FIG. 1A, as just described. When the upper heating element 24 is activated to heat the MoO₃ (i.e., to evaporate the metal oxide), the flow of N₂ is directed towards the bucket 32 through the inlet opening IN₁. As a result, the evaporated metal oxide (molecular clusters), generally at 34, are caused to flow towards the reacting zone RZ along the distal end of the center tube 10 a. The reacting zone is actually divided into four sub-zones K₁, K₂, K₃ and K₄.

The molecules of the first and second reacting agents (H₂ and H₂S) are brought to the nozzles 23 by the inert carrier N₂ through the flow line defined by the tube 10 b. As a result of appropriate pressure provided by the flow rate of these gases towards the nozzles, they diffuse into the tube 10 a through the nozzles opening, thereby interacting with the molecular clusters 34. It should be understood that, because the H₂ molecules of are much lighter than those of H₂S, they diffuse quicker, and therefore, the interaction between the evaporated molecular clusters 34 and the H₂ molecules occurs prior to the interaction with the molecules of H₂S. The H₂S actually react with components resulting from the reaction between the evaporated molecular clusters 34 and H₂, i.e., the reduced metal oxide, or metal suboxide, MoO_(3−x), generally at 36. The latter condense causing the formation of nanosize clusters thereof 38. The condensation occurs as a result of the fact that at the evaporating temperature of MoO₃, its suboxide condenses.

Thus, the molecules of H₂S interact with the nanosize clusters 38, resulting in the formation of surface layers 40 (sulfide layers MoS₂ in the present example) on the clusters 38. As this reaction proceeds, further layers 42 of MoS₂ are formed. As the so-produced particles are heavier, they propagate towards the bottom part of the tube 10 b, and the reaction proceeds during this propagation, and thereafter. By this, the desired size of the so-produced IF nanoparticles is obtained.

In order to obtain the desired particles' size, it may be needed to remove the oxygen from the produced IF nanoparticles precipitating on the ceramic filter CF, and to introduce sulfur. To this end, the precipitating material is further heated to a temperature ranging between 850° C. and 950° C. for several hours (about 2–4 hours). This can be implemented by halting the supply of the reaction materials, and lifting the reactor such that the bucket 32 is located outside the zone being heated, and only the vicinity of the ceramic filter is surrounded by the heating element.

It is important to note that, although in the present example H₂S is used as the second reacting agent, any other volatile organic sulfur or selenium compound may be used, as well as the mixtures thereof.

Thus, the reduction reaction of the vapor oxide (molecular clusters 34) with hydrogen takes place in the sub-zone K₁, can be described as follows:

wherein m=1,2,3, . . . ,9 depends on the relative concentrations of (MoO₃)₃ and H₂.

The coalescence of non-volatile suboxide molecular clusters with trioxide molecular clusters is a multi-step process, which results in the formation of suboxide nanoparticles in the sub-zone K₂. This multiple step reaction can be schematically represented through an apparent single step of condensation:

wherein n=3p+3s are integers and x−s*m/n.

The reduction process is being continued in sub-zone K₃ while hydrogen encounters suboxide nanoparticles, which have been formed in reactions 1+2.

The rate of reduction is proportional to the specific surface area of the nanoparticles. The reduction rate of molybdenum oxide powder decreases substantially as a result of aggregation of the nanoparticles (k₁>>k₃). Therefore, the condensation of the molecular clusters (MoO₃)₃ into nanosize particles (MoO_(3−x))_(n) inhibits their fast reduction process.

The condensed oxide nanoparticles flow to the sulfur diffusion volume, i.e., close to nozzles 23 within the sub-zone K₄. At this point, synthesis of IF-MoS₂ commences. The suboxide nanoparticles surrounded by a few sulfide shells (IF), flow to the ceramic filter CF (FIG. 1A), where they are collected and continue to sulfidize. Complete conversion of the oxide nanoparticles affords hollow IF-MoS₂ after a few hours reaction:

In order to elaborate the growth mechanism of the nanoparticles, the reaction intermediates are probed due to their precipitation in different colors (indicative of the oxide intermediates) onto the quartz substrate (rod) 30. Different reduced phases of molybdenum oxide (MoO_(3−x), MoO₂ and even metallic Mo), as well as molybdenum sulfide, were observed as a separate phases on the rod. This is illustrated in FIG. 2. The following components are adhered onto the successive locations of the rod (with respect to the direction of flow of the metal oxide towards the reacting zone): different oxidation states of molybdenum oxide between MoO_(3−x) and Mo (x varying between 0.1 and 3), and the resulting product MoS₂. The length of the rod part defined by the adherence of MoS₂ presents the H₂S diffusion length (L_(H2S)). The length of the rod part defined by the adherence of all the above components present the H₂ diffusion length (L_(H2)).

The above observations are indicative of the following mechanism, based on the known fact that molybdenum oxide is (known to exist in many reduced phases MoO_(3−x) with. 0<x≦1. As described above in FIG. 1B in the present reaction the reduction and sulfidization reactions of the oxide nanoparticles occur already in the central tube 10 a. This means that both hydrogen and H₂S gasses diffuse from the intermediate tube 10 b into the central tube 10 a against the nitrogen gas flow pressure.

After evaporation, the MoO₃ vapors are heated-up and carried towards the nozzles 23 inside the tube 10 a by the N₂ flow (with the appropriate flow rate J_(N2)). Along this path, the MoO₃ clusters are reduced by hydrogen, and they partially precipitate on the upper part of the quartz rod 30. Energy dispersion spectrometry (EDS) analysis of the precipitated components indicates that the degree of reduction of the molybdenum oxide increases towards the lower part of the tube 10 a, close to the nozzles 23, where the hydrogen concentration is higher. The reaction of MoO_(3−x) with the H₂S gas diffusing from 10 b to 10 a leads to the formation of MoS₂ and its deposition on the lower part of the rod 30. The spatial separation (within the reactor height) between the reduction and sulfidization reactions is mainly attributed to the difference in the diffusion length of hydrogen (L_(H2)) and the diffusion length of H₂S (L_(H2S)) due to the difference in the diffusion coefficients of the two gases. The diffusion coefficient D is determined as follows: D∝1/M)^(1/2), wherein M is the molecular weight of the gas (M_(H2S)=34; M_(H2)=2).

When strong hydrogen diffusion occurred (i.e., the H₂ diffusion length L_(H2) is higher than the bucket height h), the MoO₃ powder inside the bucket was reduced. On the other hand, no reduction of oxide powder inside the bucket was found when the precipitation height of the suboxide was a few millimeters below the bucket height (i.e., L_(H2)<h). Therefore, the diffusion lengths L_(H2) and L_(H2S) were determined by the precipitation height of the suboxide and the sulfide, respectively, on the quartz rod 30. The difference in these two parameters (L_(H2)−L_(H2S)) characterizes the spatial separation between the reduction and sulfidization reactions of the MoO₃ vapor and is referred to as the “reduction volume” or “reduction path”.

The above model described with respect to FIG. 1B and elaborated by the analysis of the precipitated components on the quartz rod 30 reveal that in this process the stoichiometry of the oxide precursor, synthesized in the vapor phase, is determined by the reduction and condensation reactions. Therefore, the stoichiometry of the suboxide nanoparticles depends on the relationship between the hydrogen flux (J_(H2)) and the flux of MoO₃ (J_(MoO3)). It is found that molybdenum based fullerene-like nanoparticles are obtained when the following condition is satisfied: J_(H2)/J_(MoO3)<1.

The hydrogen stream J_(H2) from the intermediate tube 10 b to the tube 10 a is determined by a diffusion, and consequently, obeys the one-dimensional Fick's laws of diffusion: j _(H2)(x,t)=−D _(H2) δC _(H2)(x,t)/δx.  (5)

Since H₂ is provided to the reaction zone by diffusion, and is consumed by first-order chemical reactions (Eqs. 1 and 3 above), the second order Fick's equation reads: δC _(H2)(t,x)/δt=D _(H2) *δ ² C _(H2)(t,x)/δx ² −k*C _(H2)(t,x)  (6) wherein j_(H2)(x,t) [mole/sec/cm²] is the flux density of the hydrogen stream; C_(H2)(x,t) [mole/cm³] is the H₂ concentration at a given location x at time t; D_(H2)[cm²/sec] is the diffusion coefficient of hydrogen; k=k₁+k₃ and kC_(H2)(t,x) is the rate of the chemical reaction for H₂.

The two counteracting processes: diffusion of H₂ into the reaction zone in the tube 10 a and its runoff by means of a chemical reaction with MoO₃, establish a steady state (δC_(H2)(t,x)/δt=0) of non-uniform (decreasing) H₂ concentration along the hydrogen diffusion path L_(H2). Such a time independent process can be described as: D _(H2)*δ² C _(H2)(x)/δx ² −k*C _(H2)(x)=0  (7)

The solution for this equation, with the boundary conditions C_(H2)(x=0)=C_(H2) ^(O) and C_(H2)(x=L_(H2))=0, yields the concentration profile of H₂: C _(H2)(x)=1.2C _(H2) ^(O)*exp [−x* (k/D _(H2))^(l/2)]−0.2 C_(H2) ^(O)*exp [x*(k/D _(H2))^(1/2)]  (8)

It should be noted that C_(H2) ⁰ is the H₂ concentration near the nozzles (23 in FIG. 1A). As will be shown below, C_(H2) ^(O) is differed from the initial H₂ concentration in the forming gas (C_(H2) ^(FG)).

Substituting L_(H2)=(D_(H2)/k)^(1/2) and the above equation 8 into the equation 5, the hydrogen flux is obtained: J _(H2) =C _(H2) ^(O) *D _(H2) /L _(H2)(1.2 exp [−x/L _(H2]+)0.2(exp [x/L _(H2)])  (9)

In the following, J_(H2) at the entrance to the nozzles 23, i.e., at x=0 is calculated and compared to J_(MoO3). L_(H2) is determined experimentally, as described above and is 2.4 cm for 5% H₂ (in the forming gas). The value of D_(H2) at the temperature of 0° C. is tabulated and is as follows: D_(H2)(0° C.)=0.688 cm²/sec. The following relationship was used to calculate the value of D_(H2) at a given T_(n): D _(H2) =D _(H2)(0° C.)x((T _(n)+273)/(273))^(1.7)  (10) from which the value 6.7 cm²/sec was obtained for D_(H2) at T_(n)=800° C.

Assuming the validity of the ideal gas relationship, C_(H2) ^(FG) at the reactor temperature (T_(n)) is: C _(H2) ^(FG)(T _(n))=C _(H2) ^(FG)(0° C.)x(273)/(T _(n)+273)  (11) from which the value 5.6*10⁻⁷ mole/cm³ was obtained for C_(H2) ^(FG) at T_(n)=800° C. and 5% hydrogen (volume percent in the forming gas). Talcing into account the cross-sectional area of the three nozzles (S_(n)=0.08 cm²), through which the diffusion occurs, the H₂ stream is calculated to be: J _(H2) =S _(n) *j _(H2)  (12)

The variation of the temperature of the nozzles (T_(n)) with the oxide evaporation temperature (T₀) is rather modest. Therefore, to a first approximation the diffusion flux of hydrogen from the intermediate tube 10 b to the central tube 10 a is independent on the temperature T₀.

Various experimental parameters affecting the reaction product were controlled in every experiment. These parameters include inter alia the following parameters: nozzles' temperature (T_(n)), oxide evaporation temperature (T₀), H₂ concentration (C_(H2)), concentration of H₂S (C_(H2S)), N₂ flow rate (J_(N2)), forming gas flow rate (J_(FG)), nozzles cross-section area (S_(n)), and bucket height (h). These are the important parameters defining the mechanism of formation of IF-MoS₂. The following is the description presenting the effect of each of these parameters (which was optimized by varying its value, while keeping the values of the other parameters constant) on the reaction products.

Referring to FIGS. 3A and 3B, there are graphically illustrated the relationship between the diffusion lengths L_(H2) and L_(H2S), and the MoO₃ vapor stream (J_(MoO3)) on the one hand, and parameters of IF-MoS₂ production (yield and, size), on the other hand, as a function of the main experimental parameters.

FIGS. 3A and 3B show the effect of N₂ gas flow rate (J_(N2)) on the diffusion lengths and IF size at different temperature conditions and the flow rate values. The hydrogen diffusion length L_(H2) could be varied from a few millimeters (from the nozzles 23) to about 90 mm, which is the height of the bucket (h). The parameter that influenced L_(H2) (and L_(H2S)) mostly was the N₂ flow rate J_(N2). Increasing the J_(N2) led to the decrease of L_(H2) (and L_(H2S)), and to a decrease of the IF nanoparticle sizes. When J_(N2) is very large (48 ml/min), the hydrogen diffusion length-L_(H2) is rather small. Consequently the reduction volume, as expressed by the reaction path ΔL=L_(H2)−L_(H2S) is rather small (<6 mm). The small value of ΔL does not permit a sufficient reduction of the oxide and condensation of the suboxide nanoparticles. Therefore, the amount of IF nanoparticles produced under these conditions is rather small (10%) and increases as J_(N2) decreases (see FIG. 3B). A pure IF phase is obtained when ΔL is larger than 17 mm, As ΔL increases (J_(N2) decreases), the size of the IF nanoparticles increases. A larger reduction path allows for the reduced oxide clusters to encounter the suboxide nanoparticles for longer period of time and thereby to grow in size. These figures display yet another phenomenon. At a lower temperature (T₀=765° C.) and higher values of the N₂ flow rate J_(N2), increasing the J_(N2) leads to a decrease in the fraction of IF-phase in the product (FIG. 3B). At a higher temperature (T₀=785° C.) and small values of J_(N2) the yield of the IF-phase remain unchanged (100%), as shown in FIG. 3A. When L_(H2) was below 10 mm, mostly 2H—MoS₂ platelets were obtained. The values of J_(N2) had negligible influence on the evaporation rate of the MoO₃ powder under the present experimental circumstances.

The effect of the nozzle cross-section area (S_(n)) on the H₂ diffusion length (L_(H2)) can be attributed to several factors: First, according to the diffusion (Fick) law, the diffusion fluxes of hydrogen and H₂S (i.e., J_(H2) and J_(H2S)) are linearly proportional to the cross-section area of the opening (nozzles) Second, the hydrodynamic regime of the reactor defines two counter-propagating fluxes (J_(N2) and J_(FG)) towards each other at the nozzles 23. It was found that the increase of the nozzle cross-section area by a factor of 2, leads to the increase of the H₂ diffusion length L_(H2) from 10 mm to 23 mm. Nevertheless, a simple relationship between S_(n) and IF-formation was not observed, since variation in this parameter influenced many of the other experimental parameters in a non-trivial way.

Another important effect is related to the inadvertent precipitation of reduced MoO₂ in the nozzles 23, which tends to clog them and thereby disturb the reaction parameters. Careful matching of S_(n) to the kinetics of the reaction and the fluxes averted this undesirable phenomenon.

As for the hydrogen concentration (C_(H2)) in the forming gas, the increase thereof leads to the increase of the diffusion lengths L_(H2), while the diffusion length L_(H2S) remained unchanged. This is illustrated in FIG. 4. According to the diffusion. (Fick) law, the diffusion flux (i.e., J_(H2) and J_(H2S)) is linearly proportional to the concentration gradient. The relationship between C_(H2) and the fraction of IF-phase in the product was, however, found to be much more complicated. Under the specific conditions of the present series of experiments (T₀=765° C., J_(N2)=44 ml/min), the increase of the hydrogen gas concentration led to the increase of the fraction of IF nanoparticles in the product. However, at lower temperatures (T₀<720° C.), the fraction of IF nanoparticles in the product decreased with increasing C_(H2), as will be described further below.

Turning now to FIG. 5, there are shown results of a few experiments carried-out in low H₂S concentrations (as compared to 4% H₂S concentration used in the most of the experiments). This presents the effect of H₂S concentration (C_(H2S)) on the diffusion length of the gas (as determined from the height of the sulfide deposits on the quartz rod). As expected from Fick's law, a simple monotonic dependence is observed. The influence of C_(H2S) became perceptible at small J_(MoO3), when L_(H2) is low. Thus, the effect of C_(H2S) was studied under low temperature T_(o)=725 C, nitrogen flux (38 ml/min) and C_(H2)=5%. At such a low temperatures (T₀=725° C.), the fraction of IF-MoS₂ increases first, and then decreases as the concentration of H₂S is increased. Decreasing of C_(H2S) from 4% to 1.7% leads to a decrease in L_(H2S) (from 7 mm to 3.5 mm), while L_(H2) remains unchanged (12 mm). Consequently the reduction volume (L_(H2)−L_(H2S)) and the IF yield increased (from 5 to 8.5 mm and from 50% to 90%, respectively). An additional decline of C_(H2S) to 1% resulted in a depletion of H₂S in tube 10 a and the, disappearance of IF in the product. It was shown that the kinetics of the sulfidization/reduction processes on the surface of the oxide nanoparticles varies strongly with H₂S concentration. When the H₂S concentration is above a threshold value, the kinetics of the reaction allows sufficiently rapid generation of an absolutely closed spherical sulfide monolayer on the oxide surface. This sulfide monolayer averts the fusion of the oxide nanoparticles into micron-size particles and promotes the growth of concentric spherical layers, characterizing fullerene-like structures. Contrarily, for 1% C_(H2S) the kinetics of the sulfidization reaction is too slow, and, consequently, 2H platelets are obtained.

Turning now to FIG. 6, the influence of the oxide evaporation temperature T_(o) on the MoO₃ and H₂ fluxes is demonstrated. However the following should be noted. First, the temperature of the nozzles (T_(n)) was influenced to a minor extent, only by varying T₀. T_(n) varied in the range 800±10° C., while T₀ was changed between 710–810° C. during these experiments. Therefore, to a first approximation, the diffusion fluxes of H₂ and H₂S (J_(H2) and J_(H2S)) through the nozzles from the tube (flow line) 10 b to the tube 10 a are taken as temperature independent in the reaction. Contrarily, the MoO₃ vapor stream (J_(MoO3)) was very sensitive to variations in the temperature (T₀): J_(MoO3)=v/M, wherein v [g/sec] is the evaporation rate, measured by weighting the bucket before and after the experiment; M=144 g/mole is the molecular weight of MoO₃. The temperature dependence of J_(MoO3) shows an exponential behavior, and corresponds to the exponential dependence of the oxide vapor pressure on temperature (T₀). The highest value of J_(MoO3), which is obtained by the evaporation of MoO₃ powder, is a fraction of a percent of the nitrogen gas flux (J_(N2)) in the central tube 10 a. Therefore, the MoO₃ flux does not influence the hydrodynamics of the gas flow in the reactor.

In FIG. 6 the calculated values of J_(H2) are represented by straight lines showing the calculated fluxes of hydrogen in the entrance to the nozzles 23 for three different H₂ volume concentrations: 3%, 5% and 8%. For this calculation, T_(n) was taken to be 800° C. and three hydrogen volume concentrations in the forming gas were considered: 3, 5, and 8%. The hydrogen diffusion is greatly influenced by the counter propagating stream of nitrogen gas, which carries the MoO₃ clusters. The N₂ flux in the exit of the nozzles 23 is normal to the hydrogen flux. Since the nozzles are narrow, the outflow rate of the carrier gas from the tube 10 a to the tube 10 b is greatly increased.

Using the flow rates of J_(N2) in the tube 10 a and J_(FG) in the tube 10 b, and the following cross section areas of tubes 10 a and 10 b: 0.08 cm² and 2 cm², respectively, the laminar rates were calculated to be 5 cm/sec and 0.5 cm/sec, respectively.

The outflow of N₂ from the nozzles dilutes the hydrogen concentration in the forming gas by a factor of 10 (C_(H2) ^(O)=C_(H2) ^(FG)/10), if the mixing is completed instantaneously. In reality, the mixing is not ideal and therefore this factor is higher. Similar effect holds for the H₂S concentration. This overwhelming effect slows down the diffusion of hydrogen from the tube 10 b to the tube 10 a considerably. Another effect, which plays a much smaller role, is the dilution of the hydrogen gas by the nitrogen in the reduction volume. This effect reduces the effective hydrogen concentration, available for the reduction reaction by a few percents only.

The calculated hydrogen flux J_(H2)(x=0) from the above model is 1.8*10⁻⁸ mole/see for 5% H₂ in the forming gas.

Comparing between the experimentally determined J_(MoO3) and the calculated J_(H2), the following should be noted. In FIG. 6, the bisection point (T₁) of the curves J_(MoO3) and J_(H2) defines two separate zones: to the left and to the right of this point. Since the reaction between H₂ and MoO₃ molecules is very efficient, the reduction process depends on the relative fluxes of the two reactants. To the left of the bisection point T₁, including T₁, J_(H2)/J_(MoO3)≧1, and consequently the hydrogen excess results in a very efficient reduction of the molybdenum oxide clusters into pure MoO₂ (or metallic Mo) particles. In this case, the reaction with H₂S does not afford fullerene-like particles. Consequently, 2H—MoS₂ platelets are obtained in this temperature regime. To the right of T₁, J_(H2)/J_(MoO3)<1 and the amount of H₂ in the reaction zone is smaller than that of MoO₃. Condensation of the clusters to suboxide MoO_(3−x) nanoparticles with subsequent IF-MoS₂ synthesis, is preferred in this regime.

The temperature dependence of J_(MoO3) is appreciably stronger than that of J_(H2). Thus, at elevated temperatures J_(MoO3)>>J_(H2), this H₂ deficiency leads to an inefficient reduction, and consequently, inefficient formation of the suboxide nanoparticles in this region. Therefore, there exists also an upper temperature threshold (T₂) for the IF phase formation. This temperature can be determined experimentally. The temperatures T₁ and T₂ can be varied to some extent by changing the hydrogen concentration in the forming gas and the nitrogen gas flux, which goes through the tube 10 a. The temperature range for fullerene-like particles formation at different hydrogen concentrations was found experimentally: for C_(H2)=5%−T₁=725° C., T₂=785° C.; for C_(H2)=8%−T₁=735° C., T₂=790° C. T₁ and T₂ are in a good agreement with the calculated values.

FIGS. 7A and 7B show the dependencies of L_(H2S), L_(H2) and the IF particle size on the J_(MoO3) (T₀) for two N₂ fluxes, respectively. These results are much less obvious if not counterintuitive. Most strikingly in this respect is the fact that the H₂ diffusion length L_(H2) increases for increasing J_(MoO3) (FIG. 7A). This result reflects the coalescence of the partially reduced oxide clusters into larger nanoparticles (as evident from the Equation 2 above). In fact, this condensation is the most crucial step in the gas phase synthesis of IF-MoS₂. Four different temperature zones can be discerned in FIG. 7A, reflecting the variation in IF size and yield. Zone I (T₀=710–725° C.) Zone I in FIG. 7A corresponds to low and constant values of L_(H2) (and L_(H2S)). As is clear from FIG. 5, L_(H2) is about 1 cm and L_(H2S) is about 0.7 cm in this zone. This region is also characterized by a relatively small values of J_(MoO3). While 2H platelets are preferred on the left-hand side of zone I, IF-MoS₂ are obtained on the right hand side of this zone. The hydrogen flux J_(H2) was estimated from Fick's law. It is found to be on the same order of magnitude as the lowest value of J_(MoO3) (on the left-hand side of zone I). Therefore, on the left side of zone I there is sufficient hydrogen to reduce the molecular clusters into metallic Mo (m=9 in Equation 1), which reacts hastily with H₂S. Furthermore, the low density of the molybdenum oxide clusters in the reduction volume results in a low encounter probability and consequently small (<5 nm) nanoparticles. These reactions however, are unable to produce IF-MoS₂ and the formation of platelets (2H—MoS₂) is therefore initially preferred. As the flux of MoO₃ increases in zone I, the complete reduction of the oxide clusters into metallic Mo is slowed down. In addition, the density of molybdenum oxide clusters in the reduction volume increases and consequently their encounter probability becomes higher. This leads to a gradual increase in the suboxide nanoparticles size (>5 nm) and, consequently, to an increase in the fraction of the IF phase in the product, towards the right end of zone I.

Zone II is characterized by a dramatic increase of L_(H2) (from 9 to 24 mm) and gradual increase in the (IF) nanoparticles sizes (from 20 nm to 100 nm) as J_(MoO3) increases modestly from 2 to 4.10⁻⁸ mole/sec. It should be noted that no external parameter, which could lead to an increase in L_(H2), has been varied during this experiment.

The explanation for this non-trivial behavior can be found in the increasing rate of the condensation of the oxide clusters (Equation 2 above) relative to their rate of reduction (Equation 1 above). The surface area of the oxide nanoparticles, which is available for farther reduction diminishes as the particle size increases. Therefore the overall rate of reduction (Equation 3 above) goes down, and the hydrogen can propagate deeper into the tube 10 a of the reactor, i.e., increasing the diffusion length L_(H2). Thus, the diffusion length L increases while the diffusion length L_(H2S) remains almost constant, leaving more time (larger reduction volume) for the small oxide clusters to be reduced and condense onto the oxide nanoparticles. This leads to coarsening of the oxide nanoparticles prior to their encounter with the H₂S gas. The larger oxide nanoparticles are characterized also by smaller deviation from stoichiometry (smaller x in Equation 2 above), which is favorable for IF formation. Therefore, the average size of the IF nanoparticles in the product increases in this zone (FIG. 7B). The increase of L_(H2) in zone II can be attributed to the condensation reaction, which is induced by the reduction of the (MoO₃)₃ clusters.

When the H₂ diffusion length L_(H2) reaches its saturation value (Zone III, T₀=740–780° C.), large IF nanoparticles with a relatively narrow size distribution were obtained. Subsequently, a slow increase of the IF nanoparticles size with J_(MoO3) (FIG. 7B) is observed. It should be noted that for high values of J_(MoO3), the fraction of IF nanoparticles in the product decreases again (Zone IV, T₀=780–810° C.). In parallel, increasing J_(MoO3) leads to a gradual and small increase in the IF particle size, from 100±20 nm on the left hand side of zone III to 150±50 nm on the right hand side of zone III. This indicates that in this zone the rate of reactions 2 and 3 do not vary with increasing J_(MoO3) and consequently the IF size reaches it maximum value.

Zone IV of FIG. 7B is characterized by an increase of the fraction 2H—MoS₂ platelets on expense of the IF nanoparticles in the product. Also, J_(MoO3) increases exponentially, beyond a certain threshold. This effect is attributed to the large density of (MoO₃)₃ clusters in the vapor phase, which can not be effectively reduced by the hydrogen gas. Therefore, the efficacy of the condensation process of the clusters into molybdenum suboxide nanoparticles diminishes.

Turning now to FIG. 7C, there is shown that when the N₂ flow is increased from 38 to 44 ml/min, the general form of the H₂ diffusion length L_(H2) and IF yield vs. J_(MoO3), is preserved. However, increasing the N₂ gas flow leads to a reduction of the H₂ diffusion length L_(H2) and the yield, and so a pure IF phase could not be obtained under this high N₂ flux.

As is clear from FIGS. 6 and 7B, increasing the temperature inside the interval suitable for IF formation, leads to an increase in the ratio J_(MoO3)/J_(H2), which in turn leads to bigger nanoparticles size.

Since the N₂ gas flow (FIG. 7C) is in opposite direction to the H₂ flux, increasing the former will lead to a slower in-diffusion of hydrogen from the tube 10 b into the tube 10 a. This in turn leads to a decrease in L_(H2), and therefore the reduction volume shrinks and the conditions for the IF phase synthesis become less favorable. In FIG. 7A, a pure IF phase is obtained at 765° C. However, if the N₂ gas flow increases from 38 to 44 ml/min (FIGS. 7C and 3B), the IF phase constitutes only 70% of the total product. The deterioration of the reaction product is attributed to the reduced supply of H₂ into the reactor. Increasing the hydrogen concentration in the forming gas leads to an increase of the diffusion flux of hydrogen from the tube 10 b to tube 10 a and to an increase in L_(H2), which consequently leads to an increase of the IF yield to 100%, as shown in FIG. 4.

Once the reaction mechanism and the effect of the various experimental parameters were studied, rational size control of the IF nanoparticles became feasible, too. For instance, FIG. 7B shows that as J_(MoO3) (T_(O)) increases so are the nanoparticles' sizes (5–200 nm), FIG. 3A shows that as J_(N2) decreases the nanoparticles size increases (100–300 nm). The error-bar for each J_(MoO3) is indicative of the scattering of the particles' size in the particular synthesis.

The size of the fullerene-like nanoparticles was shown to be determined by the size of the suboxide nanoparticles. In turn, the size of the suboxide nanoparticles depends on the density of the oxide molecular clusters-Mo₃O₉ (J_(MoO3)) in the reduction volume and the size of the reduction volume (L_(H2)−L_(H2S)). Variation of these parameters permits a full control of the average particle size.

It is important to emphasize that the experimental parameters are not independent of each other and in fact are highly correlated. For example, in order to decrease H₂ flux (J_(H2)), one could decrease the C_(H) ₂ in the forming gas, but also to increase the N₂ flow rate or decrease S_(n). These examples illustrate the complexity of this process and explain the great difficulty to control the synthesis of IF-MoS₂.

The effect of other experimental parameters on the IF particle size is more involved and was studied to some extent. IF particles as large as 300 nm could be produced, using the present technique, beyond this size 2H platelets were favored during the growth process.

FIG. 8 shows a typical image of IF nanoparticles with two different sizes (2 nm and 20 nm) obtained with the above technique. FIG. 9 shows a Transmission Electron Microscope (TEM) image of a plurality of IF nanoparticles obtained in the reactor illustrated in FIG. 1A.

The following are some more examples of the present invention.

EXAMPLE 2

Reference is made to FIG. 10A illustrating apparatus, generally designate 2, used for production of IF-MoS₂ nanostructures. In the apparatus 2, a chemical reactor 50 is differs from reactor 10 of the previously described reactor 1 in several structural elements. The nozzles present in reactor 1 were eliminated. This was done since the nozzles clogged after several hours of reaction. Their elimination permits a longer reaction time resulting in higher yields. However, their elimination requires different control means on the diffusion process. For such a control reactor 50 comprises two additional features, an additional tube 50A and filter 55 (vide infra). Thus, the chemical reactor 50 consists of three concentric quartz tubes: spaced-apart internal tubes 50A and 50B (of, respectively, narrower and higher diameters in the present example), and an external tube 50C.

The reactor 50 presents a vertical flow line defining an evaporation zone EZ and a reacting zone RZ. The RZ starts inside tube 50B at its distal end and extends towards the gap between tubes 50A and 50B and further into the outer tube 50C, where the resulting product is collected (precipitates) on a ceramic filter 53. MoO₃ powder is maintained at the lower part of the evaporation zone EZ at tube 50B. The MoO₃ powder is kept in a small bucket 54 (in the amount of 1.5 g in the present example). The reactor 50 is associated with vessels (not shown) containing, respectively, N₂ (an inert carrier), N₂/H₂ (forming gas, containing a first reacting agent-reducing gas) and H₂S (a second reacting agent-sulfidizing agent). These materials are sequentially fed to the reactor 50 through flow controllers 5, 6, 7 and 8 via three inlet openings (IN₁, IN₂ and IN₃) for such feedings. Controller 8 controls the flow of N₂ via IN₃ towards the solid MoO₃ powder through the EZ to the RZ along tube 50B. Controllers 6 and 7 control the flows of N₂/H₂ and H₂S towards the reacting zone RZ along the path of tube 50C. Controller 5 controls the flow of H₂/N₂ (forming gas) through the inner tube 50A towards the RZ.

In the first stage of the reaction, N₂ flowing from IN₃ carries the vaporized MoO₃ particles towards the distal end of tube 50B. A mixture of H₂/N₂ flowing from IN₁ along tube 50A partially penetrates into tube 50B commencing the reaction. As explained above for the reactor described in FIG. 1A, the diffusion depth of H₂ (the reducing gas) into the stream of MoO₃ vapor carried with the aid of N₂ affects the size distribution as well of the formed nanoparticles.

The typical velocity distribution (laminar flow) in a tube prior to passage through the filter (before filter) is schematically shown in FIG. 10B. The in-depth flow of H₂ into the tube is smaller in the central region of the tube than in the peripheral regions thereof (in the vicinity of the tube walls), ultimately leading to a variations in flow that ultimately would result in a size variations of the formed nanoparticles. In order to average the velocity of flow and thereby attain to a uniform flow of H₂ into tube 50B, the filter 55 was added in the vicinity of the distal end of tube 50B.

Referring back to FIG. 10B, the velocity profile resulting from the addition of the filter 55 is shown (after filter). Generally, the diameter of tube 50A (6–10 mm) is less or equal to that of tube 50B (10 mm), and the distance between tubes 50A and 50B is varied (for example, it may vary from about 10 mm to about 50 mm). Varying such parameters as the diameter of tube 50A and the distance between tubes 50A and 50B allows for the efficient control of the diffusion of H₂ into tube 50B and therefore provides efficient control of the resulting particle size. Thus, the provision of the additional two elements in the reactor 50 compared to reactor 10, namely tube 50A and the filter 55 at the distal end of tube 50B, serve as effective control means and are efficient substitute for the rejected nozzles.

The second stage of the reaction occurs where the reduced suboxide MoO_(3−x) particles interact with the flow of H₂/N₂ and H₂S (comprising the second reacting gas) flowing through IN₂. The flow carries the formed IF-MoS₂ particles towards the ceramic filter 53 where the particles are collected and further react with H₂S. At the lower part of tube 50C, reaction gases are discharged from the reactor through an outlet opening EXIT. The geometry of the reactor 50 is shown in FIG. 10A in a self-explanatory manner.

As further shown in FIG. 10A, two heating elements 51 and 52 (constituting a temperature control means) located outside the reactor 50 are operated to provide required temperature conditions of the entire process. More specifically, the lower heating element 51 serves for maintaining the temperature conditions required for the evaporation of the metal oxide, while the upper heating element 52 serves for providing and maintaining the temperature condition required for the chemical reactions. Practically, the three-tube quartz reactor 50 was installed inside a tube like oven (i.e., two heating elements). The temperature profile of the oven was accurately determined with a precision of ±1° C. along the designed quartz reactor.

The temperature in the EZ is from about 710° C. to about 830° C., and in the RZ it is from about 650° C. to about 950° C. Flowcontroller 5 is set to send a mixture H₂/N₂ gases through tube 50A in an amount of from about 40 cc/min to about 150 cc/min (comprising from 1 to 10% of H₂). Flowcontroller 8 is set to send N₂ gas for deriving the vaporized MoO₃ particles along tube 50B in an amount of from about 20 cc/min to about 150 cc/min, and flowcontrollers 6 and 7 are set to send a mixture of H₂/N₂/H₂S in an amount of about 200 cc/min containing H₂S gas in an amount of from about 2 cc/min to about 10 cc/min.

EXAMPLE 3

FIG. 11 illustrates an apparatus 3 used for production of IF-MoS₂ nanostructures. The apparatus 3 comprises a chemical reactor 70 that consists of three concentric quartz tubes 70A, 70B and 70C. The arrangement is such that the interior of tubes 70B and 70C are connected to each other, thereby defining inner tubes 70A and 70B, and external tube 70C. This arrangement is located inside a tube-like housing 70D. The reactor 70 presents a vertical flow line defining a reacting zone RZ starting inside the inner tube 70A at its distal end and extending into tube 70B. The resulting product is collected (precipitates) on a ceramic filter 74 located at the bottom part of the tube 70B. MoO₃ powder is maintained in the tube 70A being located within an evaporation zone EZ on an additional filter 73. The reactor 70 is associated with vessels (not shown) containing, respectively, N₂ (constituting an inert carrier), H₂/N₂ (forming gas-constituting a first reacting agent) and H₂S (constituting a second reacting agent). These materials are sequentially fed into the reactor 70 through-flow controllers 12, 13 and 14 via two-inlet opening IN₁, IN₂. The controller 12 controls the flow of N₂ towards the evaporation and reaction zones EZ and RZ through the inner tube 70A, and controllers 13 and 14 control the flows of N₂/H₂ and H₂S, respectively, through the tube-like housing 70D towards the reacting zone RZ through the external tube 70C towards the inner tube 70B. It should be noted, although not specifically shown, that an additional inlet may be provided in the reactor 70 for supplying additional amount of MoO₃ powder in a continuous flow or by discrete portions thereof. Also formed in the reactor at its lower part of tube 70B is an outlet opening EXIT for discharging the reaction gases. The geometry of the reactor 70 is shown in the figure in a self-explanatory manner.

In the first stage of the reaction, N₂ flowing from IN₁ carries the vaporized MoO₃ particles towards the distal end of tube 70A. A stream of the forming gas H₂/N₂ (comprising the first reacting agent) and H₂S (comprising the second reacting agent) flows from IN₂ along the housing 70D through tube 70C towards tube 70B. However, some of these gases also undergo diffusion into the tube 70A, where H₂(first reacting agent) being lighter diffuses deeper than H₂S thereby commencing the reaction by partially reducing the evaporated MoO₃ to suboxide nanoparticles. As explained above, the diffusion depth of H₂ into the stream of MoO₃ vapor carried with the aid of N₂ affects the size of the formed nanoparticles. Due to the absence of nozzles, the gas flow is laminar (as shown in FIG. 10B), i.e., the flow rate is uneven across the tube, decreasing from the central region of the tube towards its peripheral regions. Hence, the diffusion depth is uneven, and therefore the particle size will have a rather large distribution. In the second stage of the reaction, the reduced suboxide particles are further swept out of tube 70A to the gap between tubes 70A and 70B and interfuse with the main flow of H₂S (second reacting gas) and N₂/H₂ flowing through tube 70C down towards tube 70B. The formed LF-MoS₂ particles are swept by the gas flow while the reaction of sulfidization continues, descend in tube 70B and are collected on the filter 74. The gas flow rates are the following. The flow of H₂/N₂ (forming gas) was from about 50 cc/min to about 100 cc/min (comprising from 1 to 10% of H₂), that of H₂S was from about 4 cc/min to about 20 cc/min and that of N₂ was from about 60 cc/min to about 200 cc/min.

Further provided in the reactor 70 are heating elements: upper, central and lower elements 71, 72 and 72′ (constituting together a temperature control means) accommodated outside the reactor 70. The heating elements are operated to provide required temperature conditions of the entire process. More specifically, the upper heating element 71 serves for maintaining the temperature conditions required for the evaporation of the metal oxide, while the lower and central heating elements 72 and 72′ serve for providing and maintaining the temperature condition required for the chemical reactions. Practically, the reactor 70 was installed inside a tube like oven (i.e., two heating elements). The temperature profile of the oven was accurately determined with a precision of ±1° C. along the designed quartz reactor.

The reactor 70, being double the size of the reactor 10 (Example 1), allows for higher yield (about 200 mg/hr of reaction product) as compared to that of reactor 10 (about 10 mg/hr). In order to compensate for a higher size distribution of the formed nanoparticles as compared to that of the reactor 10), and even more increase the yield of production, the reactor 70 can be modified as exemplified below.

EXAMPLE 4

FIG. 12 illustrates yet another apparatus 4, used for production of IF-MoS₂ nanostructures, utilizing a chemical reactor 60 having a somewhat different design as compared to the reactor 70. Reactor 60 has substantially the same geometry as the reactor 70, but its design provides for different gas-flow directions, and comprises an additional filter 66, the purpose of which will be described further below. Reactor 60 presents a fluidized bed reactor, comprising three concentric quartz tubes 60A, 60B and 60C arranged as described above with respect to reactor 70, and installed in a tube-like housing 60D. A reacting zone RZ starts inside the inner tube 60A at its distal end and extends into tube 60B. The resulting product is collected (precipitates) on a ceramic filter 63 located at the bottom part of the tube 60B. MoO₃ powder is maintained on a filter 64 located in the tube 60A within an evaporation zone EZ above the reacting zone RZ. Tube 60A further comprises at its distal end another filter 66 for averaging the velocity of flow and attaining to a substantially uniform flow of gas. The reactor 60 is associated with vessels (not shown) containing, respectively, N₂ (constituting an inert carrier), H₂/N₂ (forming gas-constituting a first reacting agent) and H₂S (constituting a second reacting agent). These materials are sequentially fed to the reactor 60 through flow controllers 8, 9 and 11 via two-inlet opening IN₁, IN₂. The controller 8 controls the flow of N₂ towards the evaporation and reaction zones EZ and RZ through the inner tube 60A, and controllers 9 and 11 controls the flows of N₂/H₂ and H₂S, respectively, towards the reacting zone RZ through the lower part of tube 60B. It should be noted, although not specifically shown, that an additional inlet may be provided in the reactor 60 for supplying additional amount of MoO₃ powder in a continuous flow or by discrete portions thereof. Also formed in the reactor at its lower part of tube 60D is an outlet opening EMT for discharging the reaction gases. Some small particles formed in the course of the reaction (as will be described below) are carried by the gas flow from tube 60C into tube-like housing 60D and are collected on filter 65. The geometry of the reactor 60 is shown in the figure in a self-explanatory manner.

In the first stage of the reaction, N₂ flowing from IN₁ carries, the vaporized MoO₃ particles towards the distal end of tube 60A. A stream of the forming gas H₂/N₂ (comprising the first reacting agent) and H₂S (comprising the second reacting agent) flows from IN₂ along tube 60B. H₂ (first reacting agent) being lighter than H₂S diffuses deeper into tube 60A thereby commencing the reaction by partially reducing the evaporated MoO₃ to suboxide nanoparticles. As explained above, the diffusion depth of H₂ into the stream of MoO₃ vapor carried with the aid of N₂ affects the size of the formed nanoparticles. The use of filter 66 enables obtaining even diffusion and, consequently, even particles size distribution. In the second stage of the reaction, the reduced suboxide particles are further swept out of tube 60A to the gap between tubes 60A and 60B and interfuse with the main flow of H₂S (second reacting gas) and N₂/H₂ flowing through tube 60B. The formed IF-MoS₂ particles begin slowly to descend in tube 60B under the fluidized-bed conditions towards the filter 63. The slow descendence occurs as a result of the opposite gas flowing from the bottom of tube 60B. This fluidized bed condition absolutely avoids effect of filter clogging, and thereby allows further significant increase of yield of production. At the end of the reaction, after turning off the gas flow, the formed IF-MoS₂ particles are collected on filter 63.

Further provided in the reactor 60 are heating elements: upper, central and lower elements 61, 62 and 62′ (constituting a temperature control means) accommodated outside the reactor 60. The heating elements are operated to provide required temperature conditions of the entire process. More specifically, the upper heating element 61 serves for maintaining the temperature conditions required for the evaporation of the metal oxide, while the lower and central heating elements 62 and 62′ serve for providing and maintaining the temperature condition required for the chemical reactions. Practically, the three-tube quartz reactor was installed inside a tube like oven (i.e., three heating elements). The temperature profile of the oven was accurately determined with a precision of ±1° C. along the designed quartz reactor.

It is evident from the experimental results that pure IF-MoS₂ phase with a well-defined size, relatively narrow size distribution, and high yield (more than 200 mg/hr) could be obtained by the present invention. Variations of the nitrogen flow rate (J_(N2)), control of the diffusion process (e.g., by means of nozzles opening cross section (S_(n)) or provision of filtering means), as well as the H₂ and H₂S concentrations (C_(H2) and C_(H2S)), lead to changes in H₂ and H₂S diffusion lengths (L_(H2) and L_(H2S)), which affect the production of the IF phase. These dependencies follow the changes of the flow rates J_(MoO3), J_(H2) and J_(H2S), which can be estimated from the known Fick laws. Although the correlation between the diffusion length L_(H2) (L_(H2S)) and IF formation is not simple, it can be concluded that the IF phase is obtained if the reduction volume (L_(H2)−L_(H2S)) is larger than some minimum value, which depends on the experimental conditions. The dependencies of the diffusion length L_(H2) on the flow rate J_(MoO) ₃ (at the evaporation temperature T₀), which are shown in FIGS. 7A and 7C, are intuitively non-trivial. The flux of H₂ propagates in opposite direction to that of MoO₃, and it would be expected that an increase in the MoO₃ flow rate (J_(MoO3)) would limit the diffusion of H₂ inside the tube 10 a. However, an opposite behavior was observed, which can not be explained through a linear diffusion process. 

1. A method for obtaining inorganic fullerene-like nanostructures, the method comprising evaporation of a metal oxide at predetermined temperature conditions, wherein the metal oxide is MoO₃ or WO₃, reduction of the evaporated metal oxide by a first reacting agent being hydrogen gas to obtain a suboxide of said metal, and interaction of the suboxide with at least one second reacting agent being a volatile sulfur or selenium compound or mixtures thereof, and being characterized in that: said evaporation occurs prior to the metal oxide arrival to a reacting zone, where the reduction and the interaction occur, and a flow of the evaporated metal oxide is driven from its initial location inside a reactor towards the reacting zone by means of an inert carrier; a flow of the first reacting agent and the at least one second reacting agent in the gas phase is supplied into the reacting zone, such that the agents' flow meets the evaporated metal oxide flow while in an opposite upward/downward direction with respect to the reacting zone, thereby causing the first reacting agent interaction with the evaporated metal oxide prior to that of the at least one second reacting agent, the interaction between the evaporated metal oxide and the first reacting agent causing reduction of the evaporated metal oxide into metal suboxide nanoparticles in the gas phase resulting in condensation of the suboxide nanoparticles, and the interaction between said at least one second reacting agent and the condensing metal suboxide nanoparticles causing formation of substantially pure phase of inorganic fullerene-like nanostructures.
 2. The method according to claim 1, wherein the first and second reacting agents are supplied into different subsequent sub-zones of the reacting zone with respect to the direction of flow of the evaporated metal oxide.
 3. The method according to claim 2, wherein said reduction occurs within the first sub-zone with respect to the direction of the evaporated metal oxide flow.
 4. The method according to claim 3, wherein said interaction between the condensing metal suboxide nanoparticles and the second reacting agent occurs within the subsequent sub- zone.
 5. The method according to claim 1, wherein an additional second reacting agent is used, the two reacting agents forming a mixture of volatile sulfur and selenium compounds.
 6. The method according to claim 5, wherein the second reacting agent is H₂S, the interaction between the second reacting agent and the condensing metal suboxide nanoparticles thereby causing sulfidization of the metal suboxide nanoparticles, thereby producing the inorganic fullerene-like nanostructures formed by metal sulfide nanoparticles.
 7. The method according to claim 6, wherein the reaction product of the interaction between condensing metal suboxide nanoparticles and said second reacting agent contains particles each formed by a core of a nanosize cluster of MoO_(3−x) and coatings of MoS₂.
 8. The method according to claim 1, wherein the inert carrier is N₂.
 9. The method according to claim 1, wherein the supply of the first and second reacting agents is carried out by said inert carrier.
 10. The method according to claim 1, wherein said driving of the flow of the evaporated metal oxide comprises controlling the rate of flow of the inert carrier.
 11. The method according to claim 1, wherein the provision of the first and second reacting agents in the reacting zone comprises controlling the rate of flow of the first and second reacting agents so as to define the diffusion length for each of the reacting agents.
 12. The method according to claim 1, wherein the temperature gradient within the reacting zone is in the range of 650°C.–950° C.
 13. The method according to claim 1 further comprising annealing of the produced nanoparticles so as to remove oxygen therefrom. 